Internal combustion engines commonly employ valves that govern the providing of air and fuel to the engine cylinders and the expulsion of exhaust from the engine cylinders, among other functions. Such valves are often actuated by way of valve trains that interact with cams, which are driven by the crankshaft of the engine as gears on the crankshaft drive complementary gears associated with the cams. Tappet-followers, hydraulic lifters, or other lifter-type mechanisms that interface the cams move substantially linearly toward and away from the cams as the cams rotate. In many such engines, push rods in turn couple these lifter-type mechanisms to rocker arms, which themselves are coupled to the valves. Consequently, the rotation of the cams is translated into linear motion by which the valves are opened and closed.
Depending upon the engine and operational circumstances, the valves of an engine should be opened and closed at different times. The exact valve timing settings that are appropriate for a given engine can vary depending upon a variety of factors including engine design characteristics and intended operational circumstances. With respect to some engines, it would be also desirable if the timing settings for the valves could be individually tailored for different engines during the manufacture of those engines. This would particularly be the case if the different engines were to be used in different operational circumstances. Further, in some engines, it would be desirable if the valve timing settings could be varied during operation of the engine, in response to changing operational circumstances.
Although it would be desirable if the valve timing settings of engines could be varied in these manners, internal combustion engines having the above-described design commonly are limited in terms of the manners in which and extent to which their valve timing settings can be varied. To begin with, it is usually not possible to vary the valve timing settings on an engine in the field, after its manufacture, during the engine's operation. Further, even during the manufacture of the engine (assuming engine components are not redesigned), variation of the valve timing settings is typically only possible by adjusting the angular positioning of the cams with respect to the crankshaft. This typically is achieved by changing the relative orientation of the gears that are associated with the cams with respect to the complementary gears on the crankshaft. However, because each of the teeth of the gears associated with the cams occupies a relatively significant sector on the respective gear, only relatively gross valve timing adjustments can be made in this manner. Thus, the ability to adjust the valve timing settings on internal combustion engines of the above-described design is significantly limited.
Besides being limited with respect to valve timing adjustments, internal combustion engines having the above-described valve trains have additional limitations. In particular, although lifter-type mechanisms such as tappet-followers make it possible to translate rotational movement of the cams into linear motion, the use of such mechanisms has certain drawbacks. The tappet-followers or other lifter-type mechanisms typically must have relatively wide faces that interface the cams, so that the lifter-type mechanisms are guaranteed to remain in contact with the nearest edges of the cams as the cams rotate. The faces on such lifter-type mechanisms tend to wear down over time. Further, in order to guarantee that the lifter-type mechanisms remain in contact with the cams rather than slide off of the cams, the lifter-type mechanisms are further prevented from moving, in directions other than toward and away from the cams, by being positioned within precise bores in the crankcase. Such precise bores can be expensive to manufacture.
It would therefore be advantageous if an internal combustion engine could be developed with an improved valve train design such that modifications to the valve timing settings could be more easily made. Further, it would be advantageous if the improved valve train design made it possible to make fine adjustments to the valve timing settings, rather than simply gross adjustments to those settings. Additionally, it would be advantageous if the improved valve train design alleviated the problems associated with maintaining the proper positioning of tappet-followers or other lifter-type mechanisms relative to the cams interfaced by those mechanisms.